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Two Blazhko Modulations, a Nonradial Mode, Possible Triple Mode RR Lyrae Puls

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Two Blazhko Modulations, a Nonradial Mode, Possible Triple Mode RR Lyrae Puls Mon. Not. R. Astron. Soc. 424, 649–665 (2012) doi:10.1111/j.1365-2966.2012.21244.x The complex case of V445 Lyr observed with Kepler: two Blazhko modulations, a non-radial mode, possible triple mode RR Lyrae pulsation, and more , , E. Guggenberger,1 K. Kolenberg,2 3 J. M. Nemec,4 R. Smolec,1 5 J. M. Benko,˝ 6 C.-C. Ngeow,7 J. G. Cohen,8 B. Sesar,8 R. Szabo,´ 6 M. Catelan,9 P. Moskalik,5 K. Kinemuchi,10 S. E. Seader,11 J. C. Smith,11 P. Tenenbaum11 and H. Kjeldsen12 1Institut fur¨ Astronomie, Universitat¨ Wien, Turkenschanzstrasse¨ 17, A-1180 Vienna, Austria 2Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 3Instituut voor Sterrenkunde, Celestijnenlaan 200D, B-3001 Leuven, Belgium 4Department of Physics & Astronomy, Camosun College, Victoria, British Columbia V8P 5J2, Canada 5Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland 6Konkoly Observatory, Research Center for Astronomy and Earth Sciences, PO Box 67, H-1525 Budapest, Hungary 7Graduate Institute of Astronomy, National Central University, Jhongli City, Taoyuan County 32001, Taiwan 8California Institute of Technology, Mail Stop 249-17, 1200 East California Boulevard, Pasadena, CA 91125, USA 9Departamento de Astronom´ıa y Astrof´ısica, Facultad de F´ısica, Pontificia Universidad Catolica´ de Chile, Av. Vicuna˜ Mackenna 4860, 782-0436 Macul, Santiago, Chile 10NASA Ames Research Center/Bay Area Environmental Research Institute, Mail Stop 244-30, Moffett Field, CA 94035, USA 11SETI Institute/NASA Ames Research Center, Moffett Field, CA 94035, USA 12Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark Accepted 2012 May 4. Received 2012 April 30; in original form 2012 February 23 ABSTRACT Rapid and strong changes in the Blazhko modulation of RR Lyrae stars, as have recently been detected in high-precision satellite data, have become a crucial topic in finding an explanation of the long-standing mystery of the Blazhko effect. We present here an analysis of the most extreme case detected so far, the RRab star V445 Lyr (KIC 6186029) which was observed with the Kepler space mission. V445 Lyr shows very strong cycle-to-cycle changes in its Blazhko modulation, which are caused by both a secondary long-term modulation period and irregular variations. In addition to the complex Blazhko modulation, V445 Lyr also shows a rich spectrum of additional peaks in the frequency range between the fundamental pulsation and the first harmonic. Among those peaks, the second radial overtone could be identified, which, combined with a metallicity estimate of [Fe/H] =−2.0 dex from spectroscopy, allowed us to constrain the mass (0.55–0.65 M) and luminosity (40–50 L) of V445 Lyr through theoretical Petersen diagrams. A non-radial mode and possibly the first overtone are also excited. Furthermore, V445 Lyr shows signs of the period-doubling phenomenon and a long- term period change. A detailed Fourier analysis along with a study of the O − C variation of V445 Lyr is presented, and the origin of the additional peaks and possible causes of the changes in the Blazhko modulation are discussed. The results are then put into context with those of the only other star with a variable Blazhko effect for which a long enough set of high-precision continuous satellite data has been published so far, the CoRoT star 105288363. Key words: asteroseismology – methods: data analysis – techniques: photometric – stars: individual: KIC 6186029 (V445 Lyr) – stars: individual: CoRoT 105288363 – stars: variables: RR Lyrae. 1 INTRODUCTION RR Lyrae stars, which are low-mass helium-burning stars on the horizontal branch, were long thought to be rather simple radial pulsators. They follow a period–luminosity–colour relation which E-mail: [email protected] makes them valuable distance indicators, and because of their age C 2012 The Authors Monthly Notices of the Royal Astronomical Society C 2012 RAS 650 E. Guggenberger et al. and evolutionary status, they are also used to study the formation a rich spectrum of additional modes in the region between 2 and and evolution of the Galaxy (Catelan 2009). They can oscillate in 4d−1. Both Fourier and O − C analyses are used to investigate the fundamental radial mode (type RRab), the first overtone (type the variability of the pulsation and the modulation (Section 4), RRc) or both modes simultaneously (type RRd), and their high and the results are compared to the case of CoRoT 105288363 amplitudes of up to 1.5 mag in V for RRab type stars made their in Section 6. Spectroscopy and theoretical Petersen diagrams are variability easy to discover, so they have been known since the end used to determine the fundamental parameters such as metallicity, of the 19th century. luminosity and mass (Section 5). Additionally, the new analytic Already more than a hundred years ago, however, it turned out that modulation approach for data analysis recently proposed by Benko,˝ there is an aspect to RR Lyrae stars which is not understood at all: Szabo&Papar´ o´ (2011) is applied to the data in Section 4.4. Blazhko (1907) found a ‘periodic change in the period’ of RW Dra, which he could not explain, and which still remains unexplained 2 BACKGROUND INFORMATION ON V445 LYR today. Shapley (1916) later found in his observations of RR Lyrae that the brightness of the maxima and the light-curve shape also V445 Lyr, with the coordinates RA 18h58m26s and Dec. 41◦3549 show periodic changes. With increasing data quality in the recent (J2000), is also known as KIC 6186029, or GR244, and has a Ke- past, the unsolved problem got even more severe, as it turned out pler magnitude of Kp = 17.4. Two publications from the pre-Kepler that not just a rather small fraction of exceptional RR Lyrae stars era exist for this target: Romano (1972) found it to be variable and were affected, but probably around 40–50 per cent of all RRab stars classified it as an RR Lyrae, and Kukarkin et al. (1973) included (Jurcsik et al. 2009; Benko˝ et al. 2010; Kolenberg et al. 2010). Also it into his name list of variable stars. Romano (1972) also lists the among RRc type stars, amplitude and phase modulation was found photographic brightnesses of maximum and minimum to be 15.3 to be surprisingly widespread (Arellano Ferro et al. 2012). and 17.3 mag, respectively, indicating a surprisingly large amplitude This so-called Blazhko effect was long thought to be a peri- of 2 mag, which is much higher than the amplitude observed even odic/regular phenomenon. Traditionally, only one Blazhko period during extreme Blazhko maxima in the modern data. This might at was assigned to each modulated star, and the phenomenon was ex- least partly be explained, however, by the difference between the pected to repeat in every Blazhko cycle, agreeing with the widely observed bandpasses. Unfortunately, no details of the observations used definition that ‘the Blazhko effect is a periodic amplitude and no light curves are given, and the forthcoming paper that was and/or phase modulation with a period of several tens to hundreds of announced by the author could not be found. We therefore cannot pulsation periods’. There were several reports about changes in the know if the observed amplitude of 2 mag is real or it is possibly due Blazhko modulation of various stars (see section 5 of Guggenberger to some observational errors. No error estimations of the observa- et al. 2011 for a recent summary), but those reports usually had to tions were given by Romano (1972). rely on sparse data with large gaps, so that it was impossible to say Since Kepler data have become available, two more publications when exactly a change took place and whether it happened continu- have dealt with V445 Lyr, both presenting the Kepler data up to ously or abruptly. The Blazhko effect was therefore still considered Q2: Szabo´ et al. (2010) listed V445 Lyr as a possible candidate for to be a strictly repetitive phenomenon with only some rare excep- the period-doubling phenomenon, and Benko˝ et al. (2010) already tions showing secondary modulation periods (e.g. CZ Lac; Sodor´ noted changes in the Blazhko modulation of V445 Lyr and reported et al. 2011) or changes on very long time-scales. It was not until the presence of radial overtones. the availability of ultraprecise data from space missions like CoRoT that strong and irregular cycle-to-cycle changes of a Blazhko star 3 KEPLER PHOTOMETRY were documented and that it became obvious that seemingly chaotic phenomena need to be accounted for when modelling the Blazhko The Kepler space mission was launched on 2009 March 6 into an effect. Earth-trailing heliocentric orbit (Koch et al. 2010). Its primary pur- While the detection of cycle-to-cycle changes in the Blazhko pose is the detection of Earth-sized planets in the habitable zone modulation posed a significant challenge for all classical models of solar-like stars through the transit method, which requires con- that required a clock-work-like behaviour, some new ideas were tinuous and ultraprecise photometry of over 150 000 stars for at published. Stothers (2006) suggested that transient small-scale mag- least 3.5 years. This is also the duration of the primary mission. netic fields modulate the turbulent convection inside the helium and Kepler therefore not only provides the longest continuous data sets hydrogen ionization zones, a mechanism which certainly could ex- ever observed for RR Lyrae stars, but also does so with the high- plain subsequent Blazhko cycles of different strengths. This sce- est photometric precision ever obtained, as a consequence greatly nario, however, was recently tested on the basis of hydrodynamical improving our knowledge about stellar pulsations.
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